Semantic artefacts (SA) are key for the description of data and for making data FAIR (findable, accessible, interoperable and reusable) [1]. SA is a broader term to include ontologies, terminologies, taxonomies, thesauri, vocabularies, metadata schemas and semantic standards. Describing SAs is fundamental to make them FAIR themselves. The Metadata for Ontology Description and Publication (MOD) was developed to provide the vocabulary required to describe ontologies, and Semantic Artefacts in general. The Data Catalogue Vocabulary (DCAT) [2] was designed to describe datasets and resources that can be catalogued. This milestone presents a DCAT-based standard for description of Semantic Artefacts and their catalogues, building on the MOD vocabulary as well as recommendations and outcomes from the FAIRsFAIR project and the Research Data Alliance Vocabulary and Semantic Services Interest Group (RDA VSSIG). This milestone also makes a distinction between the MOD specification and a series of mappings with other vocabularies, presented in a machine-actionable way. Last but not least, we describe MOD profiles and how to formalise them in a machine-actionable and composable way. The next step (deliverable D4.3) related to this milestone will be to specify a common Application Programming Interface (API) for interoperability of SA catalogues in the European Open Science Cloud (EOSC) ecosystem and beyond, building from MOD descriptions of SAs. The API for SA-catalogues will enable interoperability and unified access to their content, enabling seamless querying and use by stakeholders independent of domain. The API will be adopted by FAIR-IMPACT T4.2’s use case SA-catalogues and it will be publicly available for other catalogues to deploy; via the API, other registries could consume content from multiple SA-catalogues. The implementation of this API will be the topic of an upcoming FAIR-IMPACT Open Call.